“ycao” <email@example.com> wrote in message
“Rennie Allen” <> firstname.lastname@example.org> > wrote in message
news:> 3D8876FB.email@example.com> …
I have a question here:
QNX’s message passing across the net isn’t fast enough to build a
predictable hard realtime net.
As Rennie said, speed as nothing to do with predicatbility nor realtime.
Yes it is. I’ve done it. It might not be fast enough for some
particular application you have in mind, but it is predictable, and is
therefore hard real-time. Real-time, in general, has no relationship to
speed, only to predictability.
Ethernet with less then 2 computers connected via cross cable is in my
opinion not predictable from one computer point of view.
I test the performance of QNX’s message passing across the net with the
I/O output Card and the mutlti-channel oscillograph. The data of the
shows that most of the time the performace is stable , but it may be
Well that is not a very good test, even with a predictable medium you may
get “unstable” througput, they are lots of variable to consider, like other
processes running on the machines.
The network card are 3C905B.That is i connect the nodes through Ethernet.
This may bring collision.
Depends how you configure the network.
I think that even i connect the nodes with the FDDI , the performence
become more predictable . But the average is not better than the
Ethernent ( Now the message is just 100bytes orso).
100 bytes is actually pretty bad since you are not using the full packet
size, hence you will NEVER get near the theoretical throughput. Each
ethernet packet has close to 30 bytes of overhead. Thus to send a packet of
100 bytes ethernet will have to send 130 bytes, that is not a very efficient
ratio. If you send 1500 bytes the 30 bytes overhead is far less
significant. If you want to reduce the handkshaking (thus likely hood of
collision) you can look at netraw or UPD and come up with a protocol
optimized for your application.
But I want the communication faster and stabler!
You can probably do so with shared memory type of devices, but these are a
LOT more expensive then networkcard. You are talking 3000$ US dollars here,
what is RapidIO?